Update: OK, it’s not so bad as it sounded. Here’s the real account, which doesn’t say exactly how to do it, but gives enough clues that it’s not too hard to figure out the rest.
As you might have guessed if you know PDF, it uses the PDF Launch Action. The PDF specification actually doesn’t mandate any safety features in the Launch Action; if you implemented a PDF reader that automatically launched anything a PDF document told you to, you’d be within the spec. But Adobe Reader, exercising normal common sense, prompts the user for permission to launch. The trick is just that the text which describes the application to be launched can be modified. The user still gets a stern warning not to launch anything untrusted.
This trick will doubtless catch some people, as even simpler tricks do (just saying “don’t worry, it’s safe” in the document itself will trick a rather large number of fools). But it isn’t really anything to get hugely worried about.